Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
Semiconductor memories generally fall into two groups: volatile and nonvolatile. Volatile memories are typically characterized by their fast access for both read and write operations. However, the content of volatile memories is lost when power is removed. Nonvolatile memories can often have read access as fast, or nearly as fast, as volatile memories, but write operations are generally slower. The advantage of nonvolatile memories is that their contents are retained, sometimes on the order of years, without power.
One type of nonvolatile memory is formed of a field-effect transistor (FET) having a floating gate capable of holding a charge. The presence or absence of stored charge on the floating-gate alters the threshold voltage (Vt) of the transistor. Floating gates are generally formed of polysilicon. To retain the charge for extended periods, the floating gate is isolated from surrounding structures by an insulator or dielectric material. The floating gate is isolated from the channel region of the transistor by a gate dielectric layer and from a conductive control gate by an intergate dielectric layer. To maintain the nonvolatile nature of the device, charge leakage from the floating gates of typical flash memory devices must be extremely low.
Other one-transistor memory cells are also known, such as ferroelectric memory cells. Ferroelectric memories exploit the properties of ferroelectric materials. These materials are useful in semiconductor memories as they have characteristics to provide a nonvolatile memory function; after a ferroelectric material has been polarized in one direction, it will hold that polarization for an extended time without further power input.
Ferroelectric materials have been successfully integrated into integrated circuit processes, but this integration can have some drawbacks. Ferroelectric materials having sufficient thermal stability for integrated circuit processing often include incompatible metals that must be separated from a silicon substrate. Such ferroelectric materials also tend to be strong oxygen sources, increasing the risk of undesirable oxidation of adjacent materials. Additionally, ferroelectric materials generally can only withstand a finite number of polarization reversals before their performance degrades.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate nonvolatile memory cells.